For example, an underpants-type disposable diaper includes an outer body forming a front panel and a back panel, and an inner body that includes an absorber and is fixed to the inner surface of the outer body. The front panel and the back panel of the outer body are joined together at the both sides to form a waist opening and a pair of right and left leg openings.
In the underpants-type disposable diaper, elongated resilient and elastic members such as rubber threads are fixed in an extended state at several sections of the outer body along a circumferential direction to form a stretchable structure along the waist portion to enhance the fit to the human body. In particular, diapers each including waist edge portion resilient and elastic members at the edge portion of the waist opening along the width direction and lower waist portion resilient and elastic members closer to the crotch than to the waist edge portion resilient and elastic members along the width direction are widely used due to their relatively good fit to the human body.
Basically, in such a stretchable structure, the outer body has a multi-layered structure and resilient and elastic members are built-in between layers thereof. The following forms are common in this stretchable structure: a form in which a hot-melt adhesive is applied like a sheet to one of an outer layer facing the outside of and an inner layer facing the inside of the resilient and elastic members to join the outer layer and the inner layer and fix the resilient and elastic members to the outer layer and the inner layer, or a form in which in order to cut down costs by reducing an amount of the hot-melt adhesive to be used or to improve flexibility, the hot-melt adhesive is applied to a peripheral surface over the entire length of the resilient and elastic members which is then sandwiched between the outer layer and the inner layer, thereby joining the outer layer and the inner layer and fixing the resilient and elastic members to the outer layer and the inner layer.
In addition, as an improvement of these stretchable structures, as illustrated in FIG. 18, there have been proposed stretchable structures in which two sheet materials 12H and 12S are intermittently joined together in the width direction and a direction orthogonal to the width direction to form a large number of sheet joined sections 70, and a large number of elongated resilient and elastic members 19 is arranged between the sheet materials 12H and 12S so as not to pass through the sheet joined sections 70 (so as to pass through the non-joined sections) and fix only both end portions of the resilient and elastic members 19 to both sheet materials 12H and 12S (refer to Patent Documents 1 to 3. These stretchable structures will be hereinafter also referred to as vertical intermittent joined form). According to the related art, the vertically aligned sheet joined sections 70 form vertically continuous grooves, and the sections between the grooves form large pleats 80 that swell to the same degree on the both front and back sides. The grooves improve air permeability and the pleats 80 produce excellent softness. Reference sign 75 in FIG. 18 represents a welded portion of the sheet materials 12H and 12S. Even when the sheet joined sections 70 are formed by using an adhesive, shape of the pleats 80 are similar.
However, in the prior-art technology, the pleats make fluffy or wave-shaped pleats. Thus, there is a problem that the diaper is inferior in terms of appearance and air permeability.
In addition, there has also been known formation of a large number of sheet joined sections by bonding two sheet materials intermittently in the width direction and continuously in a direction crossing the width direction (refer to Patent Document 4. The form is hereinafter referred to as a vertical continuous joined form).
However, although the prior art has excellent appearance and air permeability and the pleats extend straight, the prior art has the problems that if it is applied to a stretchable structure of a common waist edge portion where resilient and elastic members are arranged as close as possible to the edge of waist opening, hard sheet joined sections vertically continue to the edge of the waist opening, and that fine contraction wrinkles continue tightly in the vertical direction, thus forming highly rigid wavy concavities and convexities at the edge of the waist opening and deteriorating wearing feeling at the waist edge portion.